Hitters fatigue based upon their projected usage. If you have a high scoring game or two early in the season, it could wildly skew those projections, making fatigue impossible to deal with for a while. Early season dampening is an effect for the first several days that allows players to play at (or close to) 100% even when their projected usage would normally be enough to fatigue them well below 100%. It's a little bit of a grace period in dealing with fatigue to help you get the season started. Someone else might know the exact formula used.